I just finished troubleshooting a similar issue. It seems that once pfSense was installed, there was no gateway setup. We had to add the gateway, and then select it as "default", then reset the firewall state. At that point everything worked as expected.
The odd thing was that we had a 1:1 inbound mapping setup, and that worked fine without a gateway. This definitely threw off our troubleshooting path a bit.
Hopefully this helps someone.